The front end of a typical radio receiver, such as a mobile telephone for just one of many examples, typically includes one or more electronic integrated circuits, or chips, that include suitable amplifiers, filters, mixers, and other components needed to convert radio-frequency (RF) signals provided to the front end by an appropriate antenna into signals that are used by the receiver's other parts, which themselves typically include one or more chips. Each chip package has a suitable number of leads for power and input and output signals, and the chip packages are commonly disposed on one or more circuit boards.
For example, U.S. Pat. No. 6,978,125 to Lindell et al. describes a dual-band direct-conversion radio receiver, and FIG. 1 is a block diagram of the front end 100 of such a receiver. A received RF signal is supplied by an antenna 101 to a band-selection filter 102, which selects frequency bands that may be, for example, between 1805 MHz and 1990 MHz. The output of the filter 102 is supplied to a low-noise amplifier (LNA) 104, and the resulting filtered, amplified RF signal is down-converted to respective analog in-phase (I) and quadrature (Q) baseband signals by respective mixers 106, 108, which combine the filtered, amplified RF signal with respective signals from a local oscillator (LO) 110 that are 90 degrees out of phase with respect to each other. The phase-shifted LO signal can be conveniently produced by a suitable phase-shifter 112. As shown in FIG. 1, the analog I and Q baseband signals are supplied to respective filters 114, 116, and respective amplifiers 118, 120, and the resultant filtered, amplified analog signals are converted into digital signals by respective analog-to-digital (A/D) converters 122, 124. The digital I, Q signals are then provided to further processing components in the receiver, such as decoders, descramblers, de-interleavers, equalizers, combiners, etc.
The RF signal from the antenna 101 is usually single-ended, or unbalanced, which is to say that the RF signal is a voltage referenced to a known potential, such as ground. The RF signal should enter the radio chip, i.e., the chip in the receiver front end that is connected to the antenna, as a single-ended signal to save costly chip area, board area, and package leads. Nevertheless, the signals on a chip are typically differential, or balanced, signals for several reasons, such as noise immunity, cancellation of even-order non-linearity, and insensitivity to ground-lead inductance. In differential signaling, one wire carries the signal, and another wire carries the inverse of the signal, with a receiving device responding to the difference between the two wires.
Thus, a single-ended-to-differential conversion is needed in the receiver front end, preferably as close to the antenna in the signal chain as possible in order to exploit the advantages of differential signals. To convert an RF signal from single-ended to differential form on the chip before an LNA requires an on-chip balun. Some receivers like those in current mobile telephones use on-board band-select filters to perform the single-ended-to-differential conversion off-chip. Other receivers use on-chip transformers or differential inductors as baluns to perform the signal conversion.
International Publication WO 2006/085238 and WO 2006/085239, both by van der Heilden et al. for “Receiver Comprising an Amplifier”, describe an RF receiver having an amplifier with a first bipolar-transistor differential amplifier stage. A center-tapped differential inductor connects the bases of the two first-stage transistors, and a center-tapped differential inductor connects the emitters of the first-stage transistors. The center taps of the differential inductors are connected to bias sources. The latter document also describes a circuit for compensating the input impedance of the amplifier.
U.S. Pat. No. 7,039,381 to Yang et al. for “On-Chip Differential Inductor and Applications Thereof” describes uses of on-chip differential inductors in radio applications, such as receiver front-ends, and U.S. Pat. No. 7,091,814 to Kyriazidou for “On-Chip Differential Multi-Layer Inductor” describes details of on-chip differential inductor design and fabrication.
M. Rajashekharaiah et al., “A Compact 5.6 GHz Low Noise Amplifier with New On-chip Gain Controllable Active Balun”, 2004 IEEE Workshop on Microelectronics and Electron Devices, pp. 131-132 (April 2004) describes a dual-gain LNA for a direct conversion receiver. The first stage transistors are connected in a common-source single-ended configuration, and the LNA has a second gain stage that is gain-controllable, on-chip, and is an active balun for single-ended-to-differential conversion.
C.-S. Lee et al., “A Low Noise Amplifier for a Multi-band and Multi-mode Handset”, 1998 IEEE Radio Frequency Integrated Circuits (RFIC) Symposium, pp. 47-50, Baltimore, Md., USA (7-9 Jun. 1998) discusses a low noise active balun and a push-pull active matching circuit in a wideband LNA integrated circuit.
U.S. Patent Application Publication No. US 2002/0187768 by Lin for “Active Balun Circuit for Single-Ended to Differential RF Signal Conversion with Enhanced Common-Mode Rejection” describes an active balun for single-ended-to-differential RF signal conversion. The circuit includes a differential amplifier.
U.S. Pat. No. 6,366,171 to Litmanen et al. describes a single-ended-to-differential signal transformation circuit that includes a phase analysis circuit and a compensation circuit to improve the phase balance of generated differential signals.
U.S. Patent Application Publication No. US 2004/0253939 by Castenada et al. for “Integrated Circuit Radio Front-End Architecture and Applications Thereof” describes a radio receiver front-end circuit that includes a multi-tap balun and an LNA, which can be on-chip components. The balun includes a single-ended primary winding and a symmetrical multi-tap secondary winding.
U.S. Patent Application Publication No. US 2006/0103468 by Su et al. for “Single-Ended Input to Differential Output Low Noise Amplifier with a Cascode Topology” describes an LNA using a cascode topology with an objective of reduced current and area compared with prior LNAs.
M. Gordon et al., “65-GHz Receiver in SiGe BiCMOS Using Monolithic Inductors and Transformers”, 6th Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, Technical Digest pp. 265-268, San Diego, Calif., USA (18-20 Jan. 2006) discusses an integrated radio receiver including among other things an LNA and a transformer balun.
The use of transformers and differential inductors as baluns is not without problems. It is difficult to make such transformers having low signal loss, well balanced output signals, and low chip area. Any signal loss directly impairs the noise figure of the receiver, and if the output signals are not well balanced, the cancellation of even-order nonlinearity suffers. Chip area must be minimized to minimize the front end's size and cost. The use of several frequency bands in a modern mobile telephone further complicates the problem because it is desirable to use a single circuit for the several bands, and thus low loss and well balanced signals are needed over a wide frequency range.
A capacitive cross-coupling (CCC) technique can be used in RF amplifiers to improve amplifier performance, especially in common-gate and common-source transistor input stages. The CCC technique is described in, for example, W. Zhuo et al., “Using Capacitive Cross-Coupling Technique in RF Low Noise Amplifiers and Down-Conversion Mixer Design”, Proc. 26th European Solid-State Circuits Conference 2000, ESSCIRC '00, pp. 116-119, Stockholm, Sweden (19-21 Sep. 2000). Briefly stated, two cross-coupling capacitors connect the gates and sources of the two input-stage transistors. The Zhuo et al. paper shows a schematic diagram of an LNA with CCC, in which the cross-coupling capacitors are 10 picofarad (pF) poly-to-poly devices and the sources of the input-stage transistors are connected to ground by respective off-chip inductors that resonate with the gate-source capacitances and input parasitic capacitance at the frequency of interest.
U.S. Patent Application Publication No. US 2003/0042983 by Hollenbeck et al. for “Single Ended Input, Differential Output Amplifier” describes an amplifier having two CCC field-effect transistors (FETs) in a common-gate configuration that have their sources coupled through respective inductors to a source bias voltage. The inductors are not integrated on the same chip as the FETs, and the inductors are not coupled. A single-ended input RF signal is presented to the source of one of the FETs, but a resistor matching the input signal source is needed to obtain a well-balanced output signal. That necessary resistor adds noise, which renders the noise performance of the amplifier unsuitable for many applications.
These and other prior approaches to low-noise amplification and single-ended-to-differential signal conversion still suffer from drawbacks in various applications, such as receiver front ends in mobile telephones and other devices. Since the first on-chip block in the signal chain of a receiver is often the LNA, it would be beneficial to have an LNA that not only amplifies the signal but also converts it from single-ended to differential form.